The lost foam or evaporative foam process involves the placement of a foam model or pattern, usually expanded polystyrene or "styrofoam", in a container or flask, then surrounding it with dry unbonded sand which is compacted to form a mold. Molten metal is then poured into the mold thereby vaporizing the foam and filling the resulting void. The metal solidifies to form a casting of this same shape which is then removed from the flask.